Insecticidal bacteria, and methods for making and using them

ABSTRACT

The invention relates to the discovery that nucleic acid sequences comprising a BtI or BtII promoter, or a combination of a BtI and a BtII promoter, a bacterial STAB-SD sequence, and a sequence encoding proteins of the  B. sphaericus  (“Bs”) binary toxin and expressed in  B. thuringiensis  (“Bt”) or Bs cells results in production of Bs binary toxin at least 10 times that of untransformed Bs cells. The invention provides nucleic acid sequences, expression vectors, host cells, and methods of increasing the toxicity of an insecticidal bacterium by transforming the bacterium with a nucleic acid sequence of the invention. Further, the invention relates to the discovery that the Cyt1Aa1 protein of Bt subspecies  israelensis  (“Bti”) reverses resistance to Bs binary toxin in Bs-resistant mosquitoes. The invention provides Bs cells expressing Bti Cyt1Aa1 protein, and methods of reversing resistance to Bs binary toxin by co-administering the Cyt1Aa1 protein with Bs binary toxin.

CROSS-REFERENCES TO RELATED APPLICATIONS

This is a continuation of U.S. application Ser. No. 09/639,576, filed Aug. 14, 2000, the contents of which are incorporated herein by reference.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

Not applicable

BACKGROUND OF THE INVENTION

Despite advances in medical science and new drugs, malaria, filariasis, dengue and the viral encephalilitides remain important diseases of humans, with an estimated 2 billion people worldwide living in areas where these are endemic (The World Health Report—1999, World Health Organization, Geneva, Switzerland (1999)) The causative agents of these diseases are transmitted by mosquitoes, and therefore disease control methods have relied heavily on broad spectrum chemical insecticides to reduce mosquito populations. However, chemical insecticide usage is being phased out in many countries due the development of insecticide resistance in mosquito populations. Furthermore, many governments restrict use of these chemicals because of concerns over their effects on the environment, especially on non-target beneficial insects, and vertebrates through contamination of food and water supplies.

As a result of these problems, the World Health Organization is facilitating the replacement of chemical with bacterial insecticides through the development of standards for their registration and use (Guideline specifications for bacterial larvicides for public health use, WHO Document WHO/CDS/CPC/WHOPES/99.2, World Health Organization, Geneva, Switzerland (1999)). Products based on bacteria are designed to control mosquito larvae, and the two most widely used are Vectobac® and Teknar®, both of which are based on Bacillus thuringiensis subsp. israelensis. In addition, Vectolex®, a new product based on B. sphaericus has come to market recently for control of the mosquito vectors of filariasis and viral diseases. These products have achieved moderate commercial success, but their high cost and lower efficacy compared to many chemical pesticides prevents them from being used more extensively in many developing countries. Moreover, concerns have been raised about their long term utility due to resistance, which has already been reported to B. sphaericus in field populations of Culex mosquitoes in India, Brazil, and France (Sinègre, et al. First field occurrence of Culex pipiens resistance to Bacillus sphaericus in southern France, VII European Meeting, Society for Vector Ecology, 5-8 September Barcelona, Spain (1994); Rao et al., J. Am. Mosq. Control Assoc. 11: 1-5 (1995); Silva-Filha et al., J Econ. Entomot 88: 525-530 (1995)).

The insecticidal properties of these bacteria are due primarily to insecticidal proteins produced during sporulation. In Bacillus thuringiensis subsp. israelensis (Bti), the key proteins are Cyt1A (27 kDa), Cry11A (72 kDa), Cry4A (128 kDa) and Cry4B (134 kDa), whereas B. sphaericus (Bs) produces 41- and 52-kDa proteins that serve, respectively, as the toxin and binding domains of a single binary toxin (Federici et al. in Bacterial Control of Mosquitoes and Blackflies, eds.: de Barjac & Sutherland, D. J, 11-44 (Rutgers University Press, New Brunswick, N.J.) (1990); Baumann et al., Microbiot Rev. 55:425-436 (1991)).

Biochemical and toxicological differences in the Bti and Bs toxins suggested that it might be possible to construct an improved bacterium by combining their toxins into a single bacterium. Numerous attempts using this approach have been made over the past decade to create a recombinant bacterium with the desired toxicity. Several groups, for example, have introduced Bti toxin genes into Bs. For example, Bar et al., J. Invertebrate Pathol. 57:149-1 58 (1991) cloned Bti endotoxin genes into Bs 2362, but found that the biological activity was lower of the recombinant organism than that of Bti. Poncet et al., FEMS Microbiol Lett. 117:91-96 (1994) cloned the cry4B and cry11A genes of Bti into Bs 2297, and Poncet et al., Appl Environ. Microbiol. 63:4413-4420 (1997) introduced the cry11A gene into the same strain by homologous recombination. Thiéry et al., Appl. Environ. Microbiol. 64:3910-3916 (1998) introduced a Bt cyt1Ab1 gene into Bs, but reported that the level of expression of the cyt1Ab1 gene was probably too low to have any significant effect on toxicity. Servant et al., Appl Environ Microbiol. 65:3021-3026 (1999) introduced Cry11A and Cry11Ba Bt toxins in Bs 2297 by in vivo recombination, and showed that the host range could thereby be increased. Bourgouin et al., Appl. Environ Microbiol. 56:340-344 (1990) introduced Bs toxin into Bti, but found no synergistic or additive effect between the toxins against mosquito larvae. Attempts to combine the advantages of Bs and Bt in other manners have also apparently not proven commercially useful. Simply growing cultures of Bs and Bt and then combining the two organisms, for example, is not effective because the spores of the two organisms are considered to form too large a proportion of the resulting mix in proportion to the weight of the toxins to provide adequate toxicity.

Commercial development of new biopesticides is costly, in part because of EPA regulations requiring extensive testing, and margins are low relative to, for example, pharmaceutical agents. It does not appear that any of the recombinant organisms reported in the past decade have shown sufficient improvement over current commercial Bti or Bs strains to warrant development for commercial use in mosquito control.

SUMMARY OF THE INVENTION

The invention provides nucleotide sequences, expression vectors, host cells and methods for achieving the high level expression of Bs binary toxin, particularly in cells of Bacillus species and especially in Bs and in Bt cells.

In particular, the invention provides nucleic acid sequences comprising, in the following order, a B. thuringiensis promoter selected from the group consisting of a BtI promoter, a BtII promoter, and a combination of a BtI and a BtII promoter, a bacterial STAB-SD sequence, a ribosome binding site, and a sequence encoding one or both proteins of a B. sphaericus binary toxin. In some embodiments, the bacterial STAB-SD sequence is selected from the group consisting of GAAAGGAGG (SEQ ID NO:1), GAAGGGGGG (SEQ ID NO:2), GAGGGGGGG (SEQ ID NO:3), GAAAGGGGG (SEQ ID NO:4), GAAAGGAGG (SEQ ID NO:5), and GAAAGGGGT (SEQ ID NO:6). The B. thuringiensis promoter is a cry promoter, and in particular can be a cry1 promoter. Further, the B. thuringiensis promoter can be cry1Aa1, cry1Aa2, cry1Aa3, cry1Aa4, cry1Aa5, cry1Aa6, cry1Ba1, cry1Ba2, cry1Ca1, cry1Ca2, cry1Ca3, cry1Ca4, cry1Ca5, cry1Ca6, cry1Ca7 cry1Fa1, cry1Fa2, cyt1Aa1, cyt1Aa2, cyt1Aa3, or cyt1Aa4. In some preferred embodiments, the B. thuringiensis promoter is a cyt1Aa1 promoter. The nucleic acid can have both a BtI promoter and a BtII promoter, and the two promoters can be overlapping.

The invention further provides expression vectors comprising the nucleic acid described above, and host cells comprising the expression vectors. The host cells can further comprise a 20 kD protein encoded by the Bti cry11A operon. In preferred embodiments, the host cell is a B. thuringiensis cell or a B. sphaericus cell.

The invention further provides a nucleic acid sequence comprising, in the following order, a B. thuringiensis promoter which binds a sigma factor A protein, a bacterial STAB-SD sequence, a ribosome binding site, and a sequence encoding one or both proteins of a B. sphaericus binary toxin.

The invention also relates to a method of enhancing production of B. sphaericus binary toxin in a host bacterial cell, said method comprising: transforming the host cell with a gene comprising, in the following order, a B. thuringiensis promoter selected from the group consisting of a BtI promoter, a BtII promoter, and a combination of a BtI and a BtII promoter, a bacterial STAB-SD sequence, a ribosome binding site, and a sequence encoding one or both proteins of a B. sphaericus binary toxin; and expressing said gene in the host cell; whereby expression of said gene enhances production of B. sphaericus binary toxin as compared to production of B. sphaericus binary toxin in a wild-type B. sphaericus cell that is not transformed with said gene. The bacterial STAB-SD sequence used in the method can be selected from the group consisting of GAAAGGAGG (SEQ ID NO:1), GAAGGGGGG (SEQ ID NO:2), GAGGGGGGG (SEQ ID NO:3), GAAAGGGGG (SEQ ID NO:4), GAAAGGAGG (SEQ ID NO:5), and GAAAGGGGT (SEQ ID NO:6). The host cell used in the method can be a B. thuringiensis cell or a B. sphaericus cell. The host cell may further express a 20 kD product of a cry11A gene.

In other embodiments, the invention relates to a method of creating a recombinant bacterium, said method comprising the steps of: transforming the recombinant bacterium with a gene comprising, in the following order: a B. thuringiensis promoter selected from the group consisting of a BtI promoter, a BtII promoter, and a combination of a BtI and a BtII promoter, a bacterial STAB-SD sequence, a ribosome binding site, and a sequence encoding one or both proteins of a B. sphaericus binary toxin; and expressing said gene in the host cell; whereby expression of said gene enhances production of B. sphaericus binary toxin as compared to production of B. sphaericus binary toxin in a wild-type B. sphaericus cell that is not transformed with said gene. The bacterial STAB-SD sequence used in the method can be selected from the group consisting of GAAAGGAGG (SEQ ID NO:1), GAAGGGGGG (SEQ ID NO:2), GAGGGGGGG (SEQ ID NO:3), GAAAGGGGG (SEQ ID NO:4), GAAAGGAGG (SEQ ID NO:5), and GAAAGGGGT (SEQ ID NO:6). The recombinant bacterium used in the method can be selected from the group consisting of B. thuringiensis, B. sphaericus, and a member of a Bacillus species other than Bti or Bs.

The invention also relates to a method of increasing toxicity of a B. thuringiensis bacterium to a mosquito, said method comprising the steps of: transforming said bacterium with a nucleic acid sequence comprising, in the following order, a B. thuringiensis promoter selected from the group consisting of a BtI promoter, a BtII promoter, and a combination of a BtI and a BtII promoter, a bacterial STAB-SD sequence, a ribosome binding site, and a sequence encoding one or both proteins a B. sphaericus binary toxin; and expressing said gene in the bacterium; whereby expression of said gene enhances production of B. sphaericus binary toxin as compared to production of B. sphaericus binary toxin in a wild-type B. sphaericus cell that is not transformed with said gene. The bacterium can further comprise a 20 kD product of the cry11A gene.

In another group of embodiments, the invention provides a recombinant cell of B. sphaericus, said cell comprising nucleic acid sequence comprising, in the following order, a B. thuringiensis promoter selected from the group consisting of a BtI promoter, a BtII promoter, and a combination of a BtI and a BtII promoter, a bacterial STAB-SD sequence, a ribosome binding site, and a sequence encoding one or both proteins of a B. sphaericus binary toxin. The bacterial STAB-SD sequence present in the recombinant cell can be selected from the group consisting of GAAAGGAGG (SEQ ID NO:1), GAAGGGGGG (SEQ ID NO:2), GAGGGGGGG (SEQ ID NO:3), GAAAGGGGG (SEQ ID NO:4), GAAAGGAGG (SEQ ID NO:5), and GAAAGGGGT (SEQ ID NO:6). The B. thuringiensis promoter can be a cry promoter, or can be selected from the group consisting of cry1Aa1, cry1Aa2, cry1Aa3, cry1Aa4, cry1Aa5, cry1Aa6, cry1Ba1, cry1Ba2, cry1Ca1, cry1Ca2, cry1Ca3, cry1Ca4, cry1Ca5, cry1Ca6, cry1Ca7 cry1Fa1, cry1Fa2, cyt1Aa1, cyt1Aa2, cyt1Aa3, and cyt1Aa4. In a preferred embodiment, the B. thuringiensis promoter is a cyt1Aa1 promoter. The recombinant cell can further express a 20 kD product of a cry11A operon.

In yet another set of embodiments, the invention provides a method for increasing toxicity of a B. sphaericus cell, said method comprising transforming the cell with a nucleic acid sequence comprising, in the following order, a B. thuringiensis promoter selected from the group consisting of a BtI promoter, a BtII promoter, and a combination of a BtI and a BtII promoter, a bacterial STAB-SD sequence, a ribosome binding site, and a sequence encoding one or both proteins of a B. sphaericus binary toxin; and expressing said nucleic acid sequence in the host cell; whereby expression of said nucleic acid sequence enhances production of B. sphaericus binary toxin as compared to production of B. sphaericus binary toxin in a wild-type B. sphaericus cell that is not transformed with said nucleic acid sequence. The bacterial STAB-SD sequence can be selected from the group consisting of GAAAGGAGG (SEQ ID NO:1), GAAGGGGGG (SEQ ID NO:2), GAGGGGGGG (SEQ ID NO:3), GAAAGGGGG (SEQ ID NO:4), GAAAGGAGG (SEQ ID NO:5), and GAAAGGGGT (SEQ ID NO:6). The B. thuringiensis promoter can be a cry promoter, or can be selected from the group consisting of cry1Aa1, cry1Aa2, cry1Aa3, cry1Aa4, cry1Aa5, cry1Aa6, cry1Ba1, cry1Ba2, cry1Ca1, cry1Ca2, cry1Ca3, cry1Ca4, cry1Ca5, cry1Ca6, cry1Ca7 cry1Fa1, cry1Fa2, cyt1Aa1, cyt1Aa2, cyt1Aa3, and cyt1Aa4. In a preferred embodiment, the B. thuringiensis promoter is a cyt1Aa1 promoter.

The invention also provides a method for suppressing resistance to a B. sphaericus binary toxin, said method comprising expressing a Bti Cyt1Aa1 protein in a B. sphaericus cell expressing said binary toxin, as well as a method for suppressing resistance to a B. sphaericus binary toxin, the method comprising expressing a Bti Cyt1Aa1 protein in a B. thuringiensis cell expressing said binary toxin. In yet another embodiment, the invention provides a method for suppressing resistance to a B. sphaericus binary toxin, the method comprising administering Bti Cyt1Aa1 protein with said binary toxin. The Bti Cyt1Aa1 protein can be in a powder of lysed, lyophilized Bti cells, or can be in the form of a purified protein. The Bti Cyt1Aa1 protein is administered in a Cyt1Aa1 protein to Bs ratio selected from about 1:2 to about 1:50. In especially preferred embodiments, the Bti Cyt1Aa1 protein is administered in a Cyt1Aa1 protein to Bs ratio of about 1:10.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1E set forth the nucleotide sequence (SEQ ID NO:7) and encoded amino acid sequence of a fragment used to clone Bs binary toxin into a plasmid. “Sigma E” and Sigma K” denote the binding sites for sigma factors E and K, respectively. The underlined sequence between nucleotides 537 and 660 denotes a portion cloned into the sequence by PCR to introduce a STAB-SD sequence, which is denoted both by capital letters in the underlined portion, and the “STAB-SD superscript. The start and stop codons of the 51.4 kD protein (SEQ ID NO:8) and of the 41.9 kD protein (SEQ ID NO:9) of Bs binary toxin are noted under the amino acid sequence. The underlined sections between nucleotides 725 and 730, and between 2245 and 2250, marked “RBS,” represents the ribosome binding sites.

DETAILED DESCRIPTION

I. Introduction

The inventions provides nucleic acid sequences, vectors, host cells, and methods for obtaining high levels of synthesis of the binary toxin of Bacillus sphaericus (“Bs”) in recombinant bacterial cells. Bs toxin is very potent, but is produced by wild-type Bs cells at low levels. This, coupled with it being only a single toxin (in contrast, for example, to Bt, which produces a complex of toxins), permits the rapid development of insects resistant to the toxin. Increasing the amount of toxin produced per cell increases the killing power of the resultant biopesticide formulation and decreases the possibility that larvae ingesting the biopesticide will survive. Moreover, the increase in amount of toxin per cell greatly increases the efficiency of the bacterial toxin fermentation production process, and reduces the amount of bacterial product that must be applied to achieve insect control. Thus, the invention markedly reduces the cost of production and use and makes biopesticides more competitive with chemical pesticides, which can be effective, but more environmentally damaging.

The nucleic acids of the invention are heterologous sequences made by inserting a STAB-SD nucleic acid sequence between a strong promoter from a Bt gene and the ribosome binding site, and combining this construct with a nucleic acid sequence encoding the binding protein of the BS toxin, or the toxin protein, or both. In preferred embodiments, both proteins are present. Surprisingly, coupling a strong Bt promoter with the STAB-SD nucleic acid sequence results in a dramatic increase in the production of the Bs toxin, by at least 10, and usually 15 to 20, times over the amount of protein produced by standard strains of unaltered (wild-type) B. sphaericus. The presence of the Bs binary toxin, in turn, results in surprising increases in the toxicity of the recombinant cells. For example, the toxicity of recombinant cells against the larvae of mosquitoes of the genus Culex is increased by more than 10 fold compared to non-recombinant cells.

Optionally, the recombinant cell further contains a 20 kD chaperone-like product of a cry11A operon. Surprisingly, the presence of this protein increases the synthesis of Bs protein from the nucleic acid sequence described above by an additional 50% to 100%, and thus increases production of the Bs toxin to some 20 to 30 times more than that produced by standard strains of Bs.

Due to the costs of obtaining regulatory approval for new pesticides and the like, it is generally desirable that the toxicity of the bacterial cells be increased by at least about 5 times against an organism of interest to warrant investment. Efforts by others for more than a decade to produce Bs toxin in Bs and Bacillus thuringiensis (“Bt”) have resulted in increases in amounts of toxin production of 2, 3, or 4-fold, too modest to be of interest for commercial production or for field use. Thus, the ability of the invention to permit production of Bs toxin in amounts that are at least 10, more usually 15, and as much as 20, 25 or even 30 times as high as that produced in standard strains of Bs is a significant and surprising advance in the art. Equally surprisingly, in tests against Culex mosquitoes, a significant vector of human disease, the toxicity of Bt cells transformed with the nucleic acids of the invention was improved by at least 10 fold, without diminishing the toxicity of the cells to other genera of mosquitoes.

Biopesticides such as Bt are produced commercially in bioreactors. The ability provided by the invention to increase the toxicity of bacterial cells such as Bt or Bs means the amount of toxin produced per unit of culture medium will be increased, permitting the culturing of smaller quantities, and a commensurately decrease in the of raw materials used for the culture medium. Thus, the invention reduces the cost of producing biopesticides, which will extend the situations in which it is cost-effective to use them in place of chemical pesticides. Moreover, the invention also provides the ability to confer Bs toxin-based toxicity on normally non-toxic bacterial species, and especially on species of bacillus which are normally non-toxic to insect larvae. Since the attributes of these other bacterial species, such as persistence in particular environments, are likely to be different than of the Bt and Bs which thus far have served as biopesticides, the invention also provides biopesticides with a different range of attributes than those currently available. The invention thereby expands the range of options for public health officials and agricultural scientists in combating insect pests.

The ability to produce high levels of Bs toxin is particularly useful to increase the toxicity of Bt subspecies, such as subsp. israelensis, which is useful to control dipteran pests such as mosquito and blackfly larvae (this strain of Bt is hereafter referred to as “Bti”), subsp. kurstaki, which is currently useful in controlling caterpillar pests, including, e.g., the corn earworm (Heliothis zia), the cabbage looper (Trichoplusia ni), and the fall army worm (Spodoptera frugiperda), and subsp. morrisoni, which is active, e.g., against coleopteran pests such as the Colorado potato beetle (Leptinotarsa decemlineata), and the cottonwood leaf beetle (Chrysomela scripta), as well as subsp. tenebrionis, and subsp. aizawai.

Moreover, the invention permits the extension of the host range of the biopesticide (that is, it extends the organisms against which the biopesticide is toxic). Bs toxin is toxic primarily to larvae of Culex and Anopheles species, while Bti is more active against Culex and Aedes species. Thus, the expression of high levels of Bs toxin in Bti cells not only increases their toxicity to Culex, but also renders the cells more useful agents against Anopheles species. Since Anopheles species are a major vector of malaria, this increased host range alone makes the invention a major addition to the public health arsenal.

The invention further relates to the discovery that the Cyt1Aa1 (also known as “Cyt1A”) protein of Bti can restore toxicity of Bs to mosquitoes that were highly resistant to Bs toxin. Other groups have previously shown that the mechanism of resistance to Bs is a loss of binding to receptors in the insect midgut. Without wishing to be bound by theory, it appears that Cyt1Aa1 allows insertion of the Bs toxin into the midgut microvillar membrane, restoring toxicity.

Working with a Culex quinquefasciatus population at least 30,000-fold resistant to B. sphaericus 2362, the strain used in commercial biopesticide formulations, combining Bti Cyt1Aa1 with B. sphaericus completely suppressed resistance. Some suppression of resistance has previously been shown with a different Cyt protein, Cyt1Ab from B. thuringiensis subsp. medellin (“Btm”). Thiery et al., Appl. Environ. Microbiol. 64: 3910-3916 (1998). Surprisingly, however, the suppression of resistance by Bti Cyt1A is several fold higher than that which was achieved with Btm Cyt1Ab. Moreover, since Bs 2362 is the strain commercially used and to which target mosquito populations have already developed significant resistance, it is particularly important to suppress resistance to this strain. Thus, the discovery that Bti Cyt1Aa1 protein restores toxicity to Bs 2362 offers a solution to a major problem which has discouraged the continued use of Bs as a biopesticide. We reported these results in Wirth et al., J. Med. Entomol. 37:401-407 (2000).

This discovery can be exploited by producing Bti Cyt1Aa1 in a bacterial cell producing Bs toxin, such as a Bs cell or a Bt cell recombinantly altered to produce Bs toxin. If the Bs toxin is produced in a Bs cell recombinantly altered to express Bti Cyt1Aa1, it is preferred to also transform the cell to express the 20 kD chaperone-like protein encoded by the cry11A operon. The sequence of cyt1Aa1 is available from GenBank under accession number X03182, and was published by Waalwijk et al., Nucl. Acids Res. 13:8207-8217 (1985). The cry11A operon and the encoded 20 kD protein are discussed further below.

Alternatively, Bti Cyt1Aa protein or Bti cells producing Cyt1Aa (or cells of other Bacillus species recombinantly altered to produce Bti Cyt1Aa) can be added to cells, granules or powder produced from Bs to render the granules toxic to organisms which would otherwise be resistant.

As shown in the Examples, below, relatively modest amounts of Cyt1Aa1 protein are sufficient to dramatically suppress or even to eliminate resistance. In preferred embodiments, the Cyt1Aa1 protein can be added to a Bs mixture in a ratio selected from 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9 or 1:10, with 1:10 being the most preferred since it affords striking reversal of resistance with relatively low amounts of added material. Higher ratios, such as 1:12, 1:15, 1:20, 1:25, 1:30, 1:35, 1:40, 1:45, 1:50, 1:55, 1:60, 1:65, or 1:70 can also be employed if, for example, it is desired to reduce the cost of adding Cyt1Aa1 protein, with the understanding that lower ratios may provide somewhat lower suppression of resistance. The assays taught in Example 5 can be used to test any particular ratio to discern if it would provide the degree of reversal of resistance desired. Ratios of Cyt1Aa1 to Bs of less than 1:100 are not preferred.

The Cyt1Aa protein can be added as purified granules; however, it is usually easier to add Cyt1Aa in the form of Cyt1Aa-producing Bti cells. Conveniently, the Bti cells are lysed and lyophilized to form a powder prior to mixing with the Bs. In preferred embodiments, the Bs is strain 2362.

Based on our results, other Cyt1 proteins from Bti will work in the same manner to reverse resistance to Bs binary toxin.

Persons of skill in the art are aware that Bs and Bt cells are generally not administered together. Without wishing to be bound by theory, this may be due to concerns that the weight of the spores produced by each species relative to the toxin may reduce the effective amount of toxin the target larvae can ingest. The modest amounts of Bti which need to be added to achieve suppression of resistance, however, remove this concern as a factor. The studies reported in the Examples show ample toxicity when Bti cells were mixed with Bs.

Bt has been used commercially as a biopesticide for some 20 years, and Bs has been used commercially for some 5 years. The use of Bti and Bs in the field has been reviewed, for example, in Mulla, M. S., “Activity, field efficacy, and use of Bacillus thuringiensis israelensis against mosquitoes,” pp. 134-160 and in Yap, H.-H., “Field trials of Bacillus sphaericus for mosquito control” pp. 307-320, in H. de Barjac and D. J. Sutherland. [eds.] Bacterial control of mosquitoes and blackflies. Rutgers University Press (New Brunswick, N.J., 1990). Persons of skill in the art are therefore familiar with growing large quantities of Bt and of Bs organisms, with formulating biopesticides from those organisms, and with applying the formulations in the field. The recombinant organisms and methods described herein can be used in any of the methods known in the art for formulating biopesticides from Bt and Bs cells.

Recombinant Bs cells of the invention can be used in any of the methods in which Bs biopesticides are currently used, but can be applied at lower application rates proportionate to their increased toxicity compared to the strain currently used commercially. For example, if the recombinant Bs has a toxicity 10 times that of the current strain, then one-tenth the weight of the material currently used can be applied to obtain the same killing power. Moreover, recombinant cells which express Cyt1Aa1 can be used against mosquito populations which have become resistant to wild-type Bs binary toxin.

Recombinant Bt cells of the invention producing Bs binary toxin can be used to control the organisms normally controlled by Bt, and in addition can be used against Anopheles species. As discussed in connection with recombinant Bs, above, recombinant Bt expressing high levels of Bs toxin can be applied at lower application rates proportionate to the increased toxicity of the recombinant to the target organism compared to the strain currently used commercially. Thus, the invention permits the use of less material. Since reducing the amount of Bt or Bs means that less Bt or Bs has to be grown, less raw material is needed to produce the same amount of killing power and thus the net cost of producing enough material to treat a given amount of area is decreased.

The sections below define terms used in this specification. They then discuss Bt and Bs bacteria and their toxins, Bt promoters suitable for use in the invention, STAB-SD sequences, the assembly of nucleic acid sequences of the invention, and the 20 kD chaperone-like protein, as well as making and using the nucleic acids, vectors, host cells and bacteria of the invention.

II. Definitions

Unless defined otherwise, all technical and scientific terms used herein have the meaning commonly understood by a person skilled in the art to which this invention belongs. The following references provide one of skill with a general definition of many of the terms used in this invention: Singleton et al., Dictionary of Microbiology and Molecular Biology (2d ed. 1994); The Cambridge Dictionary of Science and Technology (Walker ed., 1988); and Hale & Marham, The Harper Collins Dictionary of Biology (1991). Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, preferred methods and materials are described. As used herein, the following terms have the meanings ascribed to them unless specified otherwise.

“Bacillus thuringiensis,” “B. thuringiensis,” and “Bt” refer to a gram positive soil bacterium characterized by its ability to produce crystalline inclusions during sporulation. The inclusions include insecticidal endotoxins. The inclusions comprise insecticidal proteins (sometimes referred to as “crystal proteins”) encoded by genes carried on plasmids. The bacteria can be “cured” of the plasmid by growing them at raised temperatures, resulting in cells which do not produce the crystal proteins. Such bacteria are referred to as “acrystalliferous” or “crystal minus” cells.

“Bacillus sphaericus,” or “Bs” refers to a gram positive soil bacterium which also produces a parasporal crystal of proteins toxic to certain insects.

“Binary toxin” refers to the toxin produced by Bs. The toxin is comprised of two proteins, one of which serves as a binding moiety and one of which serves as the toxin. The two proteins are capable of associating in a solution to form a functional toxin.

“Cry” and “Cyt” refer to members of two families of proteins produced by B. thuringiensis. The nomenclature in the art has recently changed from referring to the various Cry proteins by Roman numerals (intended to denote the apparent ranges of organisms to which the proteins are toxic) to Arabic numerals; the Cyt proteins were also redesignated. A comprehensive table correlating the nomenclature of the older designations and the current designations for some 130 Cry and Cyt proteins is set forth in Crickmore et al., Microbiol. Mol. Biol. Rev. 62:807-813 (1998).

The protein now termed “Cyt1Aa1” was sometimes previously referred to as “CytA” or “Cyt1A;” references herein to CytA or to Cyt1A refer to Cyt1Aa1.

Following standard usage in the art, the use of the terms “Cry” or “Cyt” herein denote the protein, while the lowercase, italicized terms “cry” or “cyt” refer to the genes.

A “promoter” is an array of nucleic acid control sequences, e.g., the cry1Ac1 promoter from B. thuringiensis, that direct transcription of an associated polynucleotide, which may be a heterologous or native polynucleotide. A promoter includes nucleic acid sequences near the start site of transcription, such as a polymerase binding site. The promoter also optionally includes distal enhancer or repressor elements which can be located as much as several thousand base pairs from the start site of transcription.

A “BtI” promoter” refers to a promoter which is recognized by sigma factor -E. A “BtII promoter” refers to a promoter which is recognized by sigma factor K.

A “strong BtI or BtII promoter” refers to a promoter which, when operably linked to a nucleic acid sequence encoding a protein, and expressed in a Bt cell, results in the protein comprising 15% or more of the dry weight of the cell.

“Sigma factors” refer to proteins known to recognize particular sequences in DNA and which form part of a complex of proteins which facilitate the initiation of transcription of the DNA by RNA polymerase. As is known in the art, the sequence of the sigma factor proteins was determined from studies in B. subtilis, the proteins performing the same functions in other Bacillus species have about 80-95% sequence to the sigma factors of B. subtilis. Accordingly, the factors which in Bt perform the same role as the sigma factors of B. subtilis have slightly different sequences than those of the paradigm proteins of B. subtilis. The term “sigma factor” herein refers to proteins in Bt performing the same function as the sigma factors of B. subtilis and having about 80-95% or higher sequence homology to those proteins. To make the point that these proteins correspond, but are not necessarily identical in sequence to the B. subtilis proteins, they are also sometimes referred to herein as “sigma-like” factors or proteins.

“Polynucleotide” and “nucleic acid” refer to a polymer composed of nucleotide units (ribonucleotides, deoxyribonucleotides, related naturally occurring structural variants, and synthetic non-naturally occurring analogs thereof) linked via phosphodiester bonds, related naturally occurring structural variants, and synthetic non-naturally occurring analogs thereof. Thus, the term includes nucleotide polymers in which the nucleotides and the linkages between them include non-naturally occurring synthetic analogs. It will be understood that, where required by context, when a nucleotide sequence is represented by a DNA sequence (i.e., A, T, G, C), this also includes an RNA sequence (i.e., A, U, G, C) in which “U” replaces “T.”

“Recombinant” refers to polynucleotides synthesized or otherwise manipulated in vitro (“recombinant polynucleotides”) and to methods of using recombinant polynucleotides to produce gene products encoded by those polynucleotides in cells or other biological systems. For example, an cloned polynucleotide may be inserted into a suitable expression vector, such as a bacterial plasmid, and the plasmid can be used to transform a suitable host cell. A host cell that comprises the recombinant polynucleotide is referred to as a “recombinant host cell” or a “recombinant bacterium.” The gene is then expressed in the recombinant host cell to produce, e.g., a “recombinant protein.” A recombinant polynucleotide may serve a non-coding function (e.g., promoter, origin of replication, ribosome-binding site, etc.) as well.

A “heterologous polynucleotide sequence” or a “heterologous nucleic acid” is a relative term referring to a polynucleotide that is functionally related to another polynucleotide, such as a promoter sequence, in a manner so that the two polynucleotide sequences are not arranged in the same relationship to each other as in nature. Heterologous polynucleotide sequences include, e.g., a promoter operably linked to a heterologous nucleic acid, and a polynucleotide including its native promoter that is inserted into a heterologous vector for transformation into a recombinant host cell. Heterologous polynucleotide sequences are considered “exogenous” because they are introduced to the host cell via transformation techniques. However, the heterologous polynucleotide can originate from a foreign source or from the same source. Modification of the heterologous polynucleotide sequence may occur, e.g., by treating the polynucleotide with a restriction enzyme to generate a polynucleotide sequence that can be operably linked to a regulatory element. Modification can also occur by techniques such as site-directed mutagenesis.

The term “expressed endogenously” refers to polynucleotides that are native to the host cell and are naturally expressed in the host cell.

An “expression cassette” refers to a series of polynucleotide elements that permit transcription of a gene in a host cell. Typically, the expression cassette includes a promoter and a heterologous or native polynucleotide sequence that is transcribed. Expression cassettes may also include, e.g., transcription termination signals, polyadenylation signals, and enhancer elements.

The term “operably linked” refers to a functional relationship between two parts in which the activity of one part (e.g., the ability to regulate transcription) results in an action on the other part (e.g., transcription of the sequence). Thus, a polynucleotide is “operably linked to a promoter” when there is a functional linkage between a polynucleotide expression control sequence (such as a promoter or other transcription regulation sequences) and a second polynucleotide sequence (e.g., a native or a heterologous polynucleotide), where the expression control sequence directs transcription of the polynucleotide.

An insecticidal endotoxin refers to a family of genes encoding endotoxin proteins that exhibit insecticidal activity, also known as crystal proteins, e.g., Cry2A, Cry3A, Cry1B, Cry1C (see Hofte & Whiteley, Microbiol. Rev. 53: 242-255 (1989)). Such insecticidal endotoxins are produced by Bacillus thuringiensis and are toxic to insects, particularly insect larvae.

An “insecticidally effective amount” of an insecticidal endotoxin is a unit dose amount that provides insecticidal activity when applied to a plant, soil, or another “locus,” e.g., site or location.

The “gene encoding the cry11A operon 20 kDa protein” (20 kDa protein gene) refers to the gene in the cry11A operon that encodes a protein of approximately 20 kDa (as described in Frutos et al., Biochem. Sys. and Ecoli. 19:599-609 (1991); see Frutos et al. FIG. 4 for nucleotide and amino acid sequence).

“Enhancing production” refers to an activity of a first protein, such as the cry11A operon 20 kDa protein, that increases the net amount of a second protein, such as an insecticidal endotoxin, in a host cell.

“Competent to express” refers to a host cell that provides a sufficient cellular environment for expression of endogenous and/or exogenous polynucleotides.

The terms “identical” or percent “identity,” in the context of two or more nucleic acids or polypeptide sequences, refer to two or more sequences or subsequences that are the same or have a specified percentage of amino acid residues or nucleotides that are the same, when compared and aligned for maximum correspondence, as measured using one of the following sequence comparison algorithms or by visual inspection.

The phrase “substantially identical,” in the context of two nucleic acids or polypeptides, refers to two or more sequences or subsequences that have at least 60%, preferably 80%, most preferably 90-95% nucleotide or amino acid residue identity, when compared and aligned for maximum correspondence, as measured using one of the following sequence comparison algorithms or by visual inspection.

For sequence comparison, typically one sequence acts as a reference sequence, to which test sequences are compared. When using a sequence comparison algorithm, test and reference sequences are input into a computer, subsequence coordinates are designated, if necessary, and sequence algorithm program parameters are designated. The sequence comparison algorithm then calculates the percent sequence identity for the test sequence(s) relative to the reference sequence, based on the designated program parameters.

Optimal alignment of sequences for comparison can be conducted, e.g., by the local homology algorithm of Smith & Waterman, Adv. Appl. Math. 2:482 (1981), by the homology alignment algorithm of Needleman & Wunsch, J. Mol. Biol. 48:443 (1970), by the search for similarity method of Pearson & Lipman, Proc. Nat'l. Acad. Sci. USA 85:2444 (1988), by computerized implementations of these algorithms (GAP, BESTFIT, FASTA, and TFASTA in the Wisconsin Genetics Software Package, Genetics Computer Group, 575 Science Dr., Madison, Wis.), or by visual inspection (see generally, Current Protocols in Molecular Biology, F. M. Ausubel et al., eds., Current Protocols, a joint venture between Greene Publishing Associates, Inc. and John Wiley & Sons, Inc., (1995 Supplement) (“Ausubel”)).

Examples of algorithms that are suitable for determining percent sequence identity and sequence similarity are the BLAST and BLAST 2.0 algorithms, which are described in Altschul et al. (1990) J. Mol. Biol. 215: 403-410 and Altschuel et al. (1977) Nucleic Acids Res. 25: 3389-3402, respectively. Software for performing BLAST analyses is publicly available through the National Center for Biotechnology Information (http://www.ncbi.nlm.nih.gov/). This algorithm involves first identifying high scoring sequence pairs (HSPs) by identifying short words of length W in the query sequence, which either match or satisfy some positive-valued threshold score T when aligned with a word of the same length in a database sequence. T is referred to as the neighborhood word score threshold (Altschul et al, supra). These initial neighborhood word hits act as seeds for initiating searches to find longer HSPs containing them. The word hits are then extended in both directions along each sequence for as far as the cumulative alignment score can be increased. Cumulative scores are calculated using, for nucleotide sequences, the parameters M (reward score for a pair of matching residues; always >0) and N (penalty score for mismatching residues; always <0). For amino acid sequences, a scoring matrix is used to calculate the cumulative score. Extension of the word hits in each direction are halted when: the cumulative alignment score falls off by the quantity X from its maximum achieved value; the cumulative score goes to zero or below, due to the accumulation of one or more negative-scoring residue alignments; or the end of either sequence is reached. The BLAST algorithm parameters W, T, and X determine the sensitivity and speed of the alignment. The BLASTN program (for nucleotide sequences) uses as defaults a wordlength (W) of 11, an expectation (E) of 10, M=5, N=−4, and a comparison of both strands. For amino acid sequences, the BLASTP program uses as defaults a wordlength (W) of 3, an expectation (E) of 10, and the BLOSUM62 scoring matrix (see Henikoff & Henikoff, Proc. Natl. Acad. Sci. USA 89:10915 (1989)).

In addition to calculating percent sequence identity, the BLAST algorithm also performs a statistical analysis of the similarity between two sequences (see, e.g., Karlin & Altschul, Proc. Nat'l. Acad. Sci. USA 90:5873-5787 (1993)). One measure of similarity provided by the BLAST algorithm is the smallest sum probability (P(N)), which provides an indication of the probability by which a match between two nucleotide or amino acid sequences would occur by chance. For example, a nucleic acid is considered similar to a reference sequence if the smallest sum probability in a comparison of the test nucleic acid to the reference nucleic acid is less than about 0.1, more preferably less than about 0.01, and most preferably less than about 0.001.

A further indication that two nucleic acid sequences or polypeptides are substantially identical is that the polypeptide encoded by the first nucleic acid is immunologically cross reactive with the polypeptide encoded by the second nucleic acid, as described below. Thus, a polypeptide is typically substantially identical to a second polypeptide, for example, where the two peptides differ only by conservative substitutions. Another indication that two nucleic acid sequences are substantially identical is that the two molecules hybridize to each other under stringent conditions, as described below.

The terms “stringent hybridization conditions” or “stringent conditions” refer to conditions under which a nucleic acid sequence will hybridize to its complement, but not to other sequences in any significant degree. Stringent conditions in the context of nucleic acid hybridizations are sequence dependent and are different under different environmental parameters. Longer sequences hybridize specifically at higher temperatures. An extensive guide to the hybridization of nucleic acids is found in Tijssen, Laboratory Techniques in Biochemistry and Molecular Biology—Hybridization with Nucleic Acid Probes, Part I, Chapter 2 “Overview of principles of hybridization and the strategy of nucleic acid probe assays,” Elsevier, N.Y., (1993) (the entirety of Tijssen is hereby incorporated by reference). Very stringent conditions are selected to be equal to the T_(M) point for a particular probe. Less stringent conditions, by contrast, are those in which a nucleic acid sequence will bind to imperfectly matched sequences. Stringency can be controlled by changing temperature, salt concentration, the presence of organic compounds, such as formamide or DMSO, or all of these. The effects of changing these parameters are well known in the art. The effect on T_(m) of changes in the concentration of formamide, for example, is reduced to the following equation: T_(m)=81.5+16.6 (log Na⁺)+0.41 (% G+C)−(600/oligo length)−0.63(% formamide). Reductions in Tm due to TMAC and the effects of changing salt concentrations are also well known. Changes in the temperature are generally a preferred means of controlling stringency for convenience, ease of control, and reversibility.

III. The Bti and Bs Toxins

Bti and Bs are aerobic, gram positive sporeforming soil organisms which produce proteinaceous crystalline inclusions during sporulation. The crystals of Bt subspecies are toxic to the larvae of a wide variety of leipdopteran, coleopteran and dipteran species. The Introduction lists some of the insects against which different subspecies of Bt are currently used. Bt comprises about 90% of all biopesticides used. Agaisse and Lereclus. J. Bacteriol. 177:6027-6033 (1995).

The cry and cyt genes encode the various insecticidal proteins produced by Bt. A large number of these genes have been identified and sequenced and are well known in the art. For example, Crickmore et al., Microbiol Mol Biol Rev., 62:807-813 (1998) (the whole of Crickmore et al. is hereby incorporated by reference) provide a table setting forth the GenBank accession numbers for the sequences of over 120 Bt identified cry genes and 9 Bt cyt genes.

As might be expected from the name, the binary toxin of Bs is composed of two proteins, one of 51.4 kD and one of 41.9 kD. The nucleotide and amino acid sequences of the proteins have been known for over a decade and are reported in Baumann et al., J. Bacteriol. 170:2045-2050 (1988). The 51.4 kD protein functions as the binding domain and the 41.9 kD protein functions as the toxin domain, thus, equimolar quantities of both proteins should usually be present for toxin function. Conveniently, this can be accomplished by having the nucleic acid sequence encoding both proteins downstream of the promoter-STAB-SD construct of the invention so that both proteins are present at the same time and in approximately the same amounts. If the nucleic acid sequence of only one of the two Bs toxin proteins is placed under the control of a promoter-STAB-SD sequence of the invention, it is desirable that a sequence encoding the other Bs toxin protein be placed under the transcriptional control of a like promoter construct so that the two proteins are produced in equimolar or roughly equimolar amounts.

In a surprising development, we have found that the Cyt1A protein from Bt can restore toxicity of the toxin to larvae which have lost receptors for the binding protein of the Bs binary toxin. Accordingly, if desired, the Cyt1A protein can be co-expressed in a cell expressing the 41.9 kD toxin protein of the Bs binary toxin in place of some or all of the 51.4 kD binding domain protein and the proteins will be toxic to target larvae ingesting the cell or biopesticides made from the cell. Thus, for example, a Bt cell, such as Bti, can be transformed by introducing a nucleic acid sequence encoding the Cyt1A protein with a promoter that will express amounts roughly similar to the amount of Bs 41.9 kD protein. Because the mechanism of action may not be the same as that of the 51.4 kD binding domain protein, it appears that the amount of the Cyt1A protein need not be as closely matched to the amount of the toxin protein as would be true for the 51.4 kD Bs binding protein to achieve full toxic effect.

IV. Bt Promoters

The invention uses promoters from Bt cry or cyt genes to drive the expression of Bs toxin. These genes encode the various insecticidal proteins produced by Bt. As noted in the previous section, Table 1 of Crickmore et al., Microbiol Mol Biol Rev., 62:807-813 (1998) sets forth a convenient listing of the names and GenBank accession numbers for the sequences of 120 Bt cry genes and 9 Bt cyt genes. Persons of skill in the art will appreciate that the 5′ sequence preceding the start codon for the coding region described in the listing for each of these genes comprises the promoter region.

As persons of skill are aware, gene transcription of Cry and Cyt proteins is temporally regulated by the presence of sequences to which proteins known as sigma factors bind. The sigma factors recruit other proteins which form a complex permitting the RNA polymerase to initiate transcription of the DNA. The promoters of the cry or cyt genes are generally classified into three categories, based on the sigma factors which bind to them. They are the sigma-E promoters, the sigma-K promoters, and the sigma-A promoters.

The sigma-E promoters are also known as BtI promoters and sigma-K promoters are also known as BtII promoters (the terms “sigma-E” and “BtI” are used interchangeably herein, similarly, the terms “sigma-K” and “BtII” are used interchangeably). A number of cry genes are active during sporulation and are generally driven by BtI or BtII promoters, with BtI promoters active earlier in sporulation than are the BtII promoters. See generally, Agaisse and Lereclus, J. Bacteriol. 177:6027-6032 (1995) (“Agaisse and Lereclus 1995”). The sigma factors which recognize most cry genes are known. E.g., Agaisse and Lereclus 1995. For example, cry4A and B are recognized by BtI promoters.

A number of cry genes have two promoters, one BtI and one BtII. The combination of these dual promoters serves to extend the expression of the gene over a longer period of the sporulation process. In some cases, the two promoters overlap. Non-sporulation dependent cry genes have yet another set of promoters, which are recognized by yet another sigma factor, sigma-A. See, e.g., Agaisse and Lereclus 1995.

Any strong BtI or BtII promoter can be used in the nucleic acid sequences and methods of the invention. Dual BtI and BtII promoters, and particularly the overlapping BtI and BtII promoters, tend to be strong promoters of protein expression and are preferred forms of promoters for constructing nucleic acids of the invention. Members of the genes falling within the following groups which have dual promoters are especially preferred: cry1A, cry1B, cry11A, and cyt1Aa.

For purposes of the invention, any promoter which can drive expression of Cyt1Aa1 protein to comprise 15% or more of the dry weight of an acrystalliferrous Bti cell is considered a strong promoter. Any BtI or BtII promoter or promoter region which comprises both a BtI and BtII promoter and resulting in the protein constituting 15% or more of the dry weight of the cell constitutes an appropriate BtI or BtII promoter for purposes of the invention.

In preferred embodiments, the BtI or BtII promoter is a cry1 promoter or a cyt1Aa1 (also known as a cyt1A) promoter. Due to the close phylogenetic relationship and high sequence identity of the cry1 promoters (see, Crickmore et al., Microbiol. Mol. Biol. Rev. 62:807-813 (1998), any of the promoters of the genes designated in Crickmore et al. as a cry1 gene is considered capable of driving high levels of expression of Bs binary toxin. Particularly preferred embodiments are cry1Aa1 (formerly called cry1A(a)), cry1Ba1 (formerly called cry1B), cry1Ca1 (formerly called cry1C), and cry1Fa1 (formerly called cry1F). It should be noted that each of these genes has other closely related genes. For example, cry1Fa1 is closely related to cry1Fa2. The other members of the named cry1 gene groups designated by the same capital letter and same lower case letter are considered to be almost as preferred as the first listed gene in the group (that is, cry1Fa2 is almost as preferred as cry1Fa1).

In another particularly preferred embodiment, the promoter can be the promoter from cyt1Aa1, which comprises both BtI and BtII promoters, and is accordingly sometimes referred to as a dual promoter. Since the promoter region contains two promoters, the promoter region of this gene is also termed the “cyt1A promoters.” Based on phylogenetic analysis and sequence identity, the promoters of the other cyt1Aa genes are also sufficiently strong BtI or BtII or combined BtI and BtII promoters to be used in the compositions and methods of the invention.

The non-sporulation dependent cry gene promoters are sigma-A promoters and are not generally satisfactory, except under a modified set of conditions. Non-sporulating forms of Bt, or course, do not divert their metabolic resources to spore production, and can accumulate toxin over a longer period than can sporulating forms. In non-sporulating forms, therefore, sigma-A promoters can be used to accumulate high levels of toxin. Accordingly, BtI and BtII promoters are preferred for use in Bacillus, including Bt and Bs. In non-sporulating forms of Bacillus, sigma-A promoters may be used.

V. STAB-SD Sequences

The crystal protein mRNAs of B. thuringiensis have an average half life of 10 minutes during sporulation, whereas the average half life of other mRNAs is between 1 to 2 minutes. This long half life may be responsible in part for the very high production of crystal proteins, which can be as much as 20-30% of the dry weight of the sporulated cells.

The long half life of these proteins is related to two untranslated regions of the genes. First, there is a sequence in the 5′ untranslated region, usually found between the promoter and the coding region, which is not involved in translation initiation but which is a determinant of stability for mRNA. The sequence is a consensus Shine-Dalgarno-(“SD”) like sequence. Second, the 3′ terminal fragment of cry genes, such as cry1Aa, increases the half life of mRNA transcripts two to threefold. Agaisse and Lereclus, Mol. Microbiol., 20:633-643 (1996) (hereafter “Agaisse 1996”).

The 5′ SD-like sequence appears to be involved in stabilizing the production of the proteins. It has, accordingly, been named a “STAB-SD” sequence. Agaisse 1996. Agaisse 1996 suggests that the STAB-SD is involved in interactions with the 3′ end of 16S ribosomal RNA, and found that mutations of the STAB-SD sequence which were expected to abolish complementarity affected the stability conferred. Interestingly, the STAB-SD sequence of the protein then called cryIIIA (now called cry3A) showed putative interactions with the 3′ end of B. subtilis 16S rRNA. Thus, it appears that STAB-SD sequences are not specific for particular species of bacteria, and that the STAB-SD sequences of other Bacillus species, and of other genera of bacteria, can be used to stabilize the production of proteins in B. thuringiensis and B. sphaericus.

Agaisse 1996 reviewed databases and identified numerous examples of putative STAB-SD sequences in 5′ untranslated regions (“UTR”), including those of four cry genes from Bt, the cwp locus of B. brevis, and the inIAB locus of Listeria monocytogenes. Any of these STAB-SD sequences can be used to produce high levels of Bs toxin in Bacillus when placed between a strong Bacillus promoter and a ribosome binding site. The STAB-SD sequences identified share fairly high homology to one another.

In preferred embodiments, the STAB-SD sequence is selected from the group consisting of GAAAGGAGG (the cry3A sequence, SEQ ID NO:1), GAAGGGGGG (the cry3B sequence, SEQ ID NO:2), GAGGGGGGG (the cry3B2 sequence, SEQ ID NO:3), GAAAGGGGG (the cry3D sequence, SEQ ID NO:4), GAAAGGAGG (the cwp from B. brevis sequence, SEQ ID NO:5), and GAAAGGGGT (the in1AB from L. monocytogenes, SEQ ID NO:6). The cry3A, cry3B, cry3B2 and cry3D sequences are particularly preferred. Cry3A is the most preferred embodiment.

Other sequences with high sequence identity to one of the STAB-SD sequences set forth above and which function as a STAB-SD sequence can be used in the nucleic acids and methods of the invention. The sequence should have at least 85% sequence identity to the STAB-SD sequences set forth above. In preferred forms, the sequence has at least about 90% sequence identity, and even more preferably has about 95% or higher sequence identity. In addition, Agaisse 1996 provides guidance on changes to putative STAB-SD sequences which may deleteriously affect stability. In general, any change in a nucleotide which would abolish interaction between the STAB-SD sequence and the 3′ end of B. subtilis 16SrRNA 3′ is likely to reduce protein production and is not preferred.

Any putative nucleic acid sequence, or any desired modification to a known STAB-SD sequence, can be conveniently tested for its function as a STAB-SD protein by placing the sequence in a plasmid containing a galactosidase coding sequence following the assays and other methods taught in Agaisse 1996, or by substituting the sequence under consideration for the STAB-SD sequence in the procedure set forth in the Examples, below, and comparing the resulting protein production to the production of the same protein from the construct using the STAB-SD sequence set forth herein. Sequences which reduce production of Bs binary toxin to less than about 8 times that of wild-type Bs cells as measured by densiometric analysis of Coomassie blue-stained SDS-PAGE gels are less preferred.

VI. Assembly of Nucleic Acid Sequences of the Invention

Considerable information has developed in the art about the construction of promoters; in this context, the following discussion is offered to provide the specific information persons of skill may need to optimize placing a STAB-SD sequence between the sigma-factor binding site of a strong Bt promoter and the ribosome binding site.

FIG. 1 demonstrates an exemplary assembly of a nucleic acid sequence of the invention. In this embodiment, the cyt1A promoters comprise nucleotides 1-537. (Since the cyt1A gene contains two promoters, one a BtI promoter and the other a BtII promoter, the promoter region of the gene is sometimes referred to in the art as the “cyt1A promoters.”) The binding sites for the sigma E-like factor and the sigma K-like factor are shown with the notations “SIGMA E” and “SIGMA K,” respectively, placed over the appropriate regions, with the terms “−35” and “−10” and underlined sequences designating the specific binding sites. The underlined nucleotides with the letters “RBS” at nucleotides 726 to 730 and 2246 to 2249 denote ribosome binding sites.

The underlined nucleotides from 538 to 659 denote a sequence cloned in from the cry3A promoter. This sequence was cloned in to introduce the 9 nucleotide STAB-SD sequence; the longer sequence from the cry3A promoter was used because it is relatively more difficult to clone in a 9 nucleotide sequence. The sequence commencing at position 660 is a portion of the sequence of the binary toxin upstream of the coding region, followed by the coding region (as published by Baumann et al., J. Bacteriol. 170:2045-2050 (1988)). The particular portion upstream of the start codon at the position marked “+1” was selected simply because of the presence of a convenient restriction site. Shorter or longer portions could be used, and indeed, the entire promoter region of the binary toxin gene can be used if desired. In general, however, use of shorter sequences is preferred, not only for ease of manipulation but also to avoid the accidental inclusion of repressor sequences or the like which might happen to be present. Additionally, if a different sequence is used after the STAB-SD sequence, a ribosome binding site should be placed between the STAB-SD sequence and the start codon. Preferably, the ribosome binding site is positioned about 6 to about 10 nucleotides upstream of the start codon.

The start and stop sites of the 51.4 kD and 41.9 kD Bs binary toxin proteins are also shown.

The manner in which the elements of this exemplary sequence are joined can be varied substantially and still result in a sequence which works well in producing high levels of Bs binary toxin. In this sequence, some 121 nucleotides from the cry3A sequence were used to clone in the STAB-SD sequence. This was done simply for ease in cloning; the 9-nucleotide STAB-SD sequence can be introduced by itself. For ease in cloning, however, it is usually preferable to use a sequence which encompasses the STAB-SD sequence and which is from about 20 to about 130 nucleotides in length. The STAB-SD sequence itself can be placed anywhere from about 10 bases downstream of the sigma factor binding site to just before an RBS sequence, which in turn should be about 6 to about 10 bases upstream of the start codon. That is, all or a portion of the promoter downstream of the sigma-factor binding site can be deleted, with the understanding that if the RBS of the promoter is deleted, another RBS, such as that from the Bs binary toxin, should be placed about 6 to about 10 bases upstream of the start codon. As noted, in the sequence depicted in FIG. 1, the sequence from position 660 on are from the native Bs binary toxin gene. Any particular sequence can be readily tested by substituting it in the assays taught in the Examples to determine whether it has a deleterious or advantageous effect on toxin production.

VII. 20 kD Chaperone-Like Protein

In the methods of the present invention, host cells are transformed with a gene encoding a 20 kDa protein gene, which encodes a known protein (Frutos et al., supra; Visick & Whitely, supra), to enhance the production of Bs binary toxin. The 20 kDa protein gene can be isolated and sequenced, for example, from two subspecies of B. thuringiensis (Frutos et al., Biochem. Syst. and Ecoli. 19: 599-609 (1991)). The level of expression of the 20 kDa protein has been characterized in cells transformed with the 20 kDa protein gene. Using methods and sequence information described herein and in International Patent Application W97/39623, the 20 kDa protein gene can be isolated by those skilled in the art and used to construct recombinant expression vectors for transformation of a host cell.

The host cells transformed with the 20 kD protein gene should be competent to express Bs binary toxin. The cells may express the Bs binary toxin, or the cells may be transformed with exogenous binary toxin expression vectors. As noted earlier, the sequence of both proteins of the Bs binary toxin is known. This sequence information can be used by one skilled in the art, along with the methods described herein, to construct recombinant vectors for transformation of a host cell, such as a Bs cell, with the gene encoding the 20 kD protein. Conveniently, the gene for the 20 kD protein can be placed on the same plasmid as the nucleic acid sequence for expressing high levels of Bs toxin.

VIII. Nucleic Acids Sequences and Vectors

A recombinant expression vector for transformation of a host cell is prepared by first isolating the constituent polynucleotide sequences, as discussed herein. The polynucleotide sequences, e.g., a sequence encoding the Bs binary toxin driven by a promoter as discussed above, are then ligated to create a recombinant expression vector suitable for transformation of a host cell. Methods for isolating and preparing recombinant polynucleotides are well known to those skilled in the art. Sambrook et al., Molecular Cloning. A Laboratory Manual (2d ed. 1989); Ausubel et al., Current Protocols in Molecular Biology (1995)), provide information sufficient to direct persons of skill through many cloning exercises.

One preferred method for obtaining specific polynucleotides combines the use of synthetic oligonucleotide primers with polymerase extension or ligation on a mRNA or DNA template. Such a method, e.g., RT, PCR, or LCR, amplifies the desired nucleotide sequence (see U.S. Pat. Nos. 4,683,195 and 4,683,202). Restriction endonuclease sites can be incorporated into the primers. Amplified polynucleotides are purified and ligated to form an expression cassette. Alterations in the natural gene sequence can be introduced by techniques such as in vitro mutagenesis and PCR using primers that have been designed to incorporate appropriate mutations. Another preferred method of isolating polynucleotide sequences uses known restriction endonuclease sites to isolate nucleic acid fragments from plasmids. The genes of interest can also be isolated by one of skill in the art using primers based on the known gene sequence.

The isolated polynucleotide sequence of choice, e.g., the Bs binary toxin driven by the promoter sequence discussed above, is inserted into an “expression vector,” “cloning vector,” or “vector,” terms which usually refer to plasmids or other nucleic acid molecules that are able to replicate in a chosen host cell. Expression vectors can replicate autonomously, or they can replicate by being inserted into the genome of the host cell. Often, it is desirable for a vector to be usable in more than one host cell, e.g., in E. coli for cloning and construction, and in B. thuringiensis for expression. Additional elements of the vector can include, for example, selectable markers, e.g., tetracycline resistance or hygromycin resistance, which permit detection and/or selection of those cells transformed with the desired polynucleotide sequences (see, e.g., U.S. Pat. No. 4,704,362). The particular vector used to transport the genetic information into the cell is also not particularly critical. Any suitable vector used for expression of recombinant proteins host cells can be used. A preferred vector is pHT3101, which is an E. coli-B. thuringiensis shuttle vector (Lereclus et al., FEMS Microbiol. Lett. 60: 211-218 (1989)).

Expression vectors typically have an expression cassette that contains all the elements required for the expression of the polynucleotide of choice in a host cell. A typical expression cassette contains a promoter operably linked to the polynucleotide sequence of choice. The promoter used to direct expression of the Bs binary toxin is as described above, and is operably linked to a sequence encoding one or both of the Bs binary toxin proteins. The promoter is preferably positioned about the same distance from the heterologous transcription start site as it is from the transcription start site in its natural setting. As is known in the art, however, some variation in this distance can be accommodated without loss of promoter function. For expression of the 20 kD protein encoded by the cry11A operon, other promoters suitable for driving the expression of a heterologous gene in a host cell can be used, including those typically used in standard expression cassettes, e.g., the β-galactosidase promoter. In one embodiment of the invention, the 20 kD protein gene is operably linked to the BtI and BtII promoters (“the cryIAc promoter”) of the cryIAc gene, creating a heterologous nucleic acid operably linked to a promoter. The cryIAc promoter is highly active in growth conditions that induce sporulation.

IX. Expression of Protein

After construction and isolation of the recombinant expression vector, it is used to transform a host cell for expression of Bs binary toxin. The particular procedure used to introduce the genetic material into the host cell for expression of a protein is not particularly critical. Any of the well known procedures for introducing foreign polynucleotide sequences into host cells can be used. Transformation methods, which vary depending on the type of host cell, include electroporation; transfection employing calcium chloride, rubidium chloride calcium phosphate, DEAE-dextran, or other substances; microprojectile bombardment; lipofection; infection (where the vector is an infectious agent); and other methods (see generally Sambrook et al., supra; Ausubel et al., supra). In some embodiments, the host cells can be transformed by homologous recombination, as described in Poncet et al., Appl Environ Microbiol. 63:4413-4420 (1997). A preferred method of transforming B. thuringiensis is electroporation, as described in Wu et al., Mol. Microbiol. 13: 965-972 (1994).

Hosts for transformation with the Bs binary toxin gene include any suitable host bacterial cell competent to express the protein, especially members of the genus Bacillus. In particularly preferred embodiments, the cells are Bs or Bt cells. Hosts that are transformed with the Bs binary toxin are useful recombinant bacteria as insecticides. Preferred subspecies of B. thuringiensis include, e.g., B. thuringiensis subsp. kurstaki, B. thuringiensis subsp. aizawai, B. thuringiensis subsp. israelensis, and B. thuringiensis subsp. tenebrionis. A preferred strain is Bti IPS82.

After the host cell is transformed with the Bs binary toxin gene, the host cell is incubated under conditions suitable for expression of the toxin. Typically, the host will be grown under conditions that promote sporulation and expression of insecticidal endotoxin genes. Host cells may be prepared in any quantity required by fermenting an inoculum in standard media known to those skilled in the art. The media will, for example, generally contain a nitrogen source and a carbohydrate source, e.g., glucose. Suitable conditions for incubation include a temperature in the range of 15-45° C., preferably 30° C., and an approximately neutral pH. Incubation may be conveniently carried out in batches, typically for a period of 3-5 days.

Various media for growing Bt and Bs cells are known in the art. In some preferred embodiments, an inoculum from a stock host cell culture is grown on nutrient agar (BBL Microbiology Systems) or peptonized milk (1% peptonized milk [BBL Microbiology Systems], 1% dextrose, 0.2% yeast extract, 1.216 mM MgSO₄, 0.072 mM FeSO₄, 0.139 mM ZnSO₄, 0.118 mM MnSO₄) with erythromycin at a concentration of 25 μg/ml, as described in the Examples.

Enhanced production of Bs binary toxin is observed after host cells competent to express the Bs binary toxin gene is transformed with the gene and the cells are grown under suitable conditions. Enhanced production of Bs binary toxin may be observed by standard methods known to those skilled in the art. For example, parasporal inclusions of insecticidal endotoxins can be purified (see Wu & Federici, Appl. Microbiol. Biotechnol. 42: 697-702 (1995) (hereafter “Wu and Federici 1995”), harvested by centrifugation from lysed cultures, or examined with microscopy (see Wu & Federici 1995, supra). Parasporal inclusions that have been harvested by centrifugation or purified may be separated using standard methods known in the art, for example, chromatography, immunoprecipitation, ELISA, bioassay, western analysis, or gel electrophoresis (see, e.g., Wu & Federici 1995, supra; Ausubel, supra). Amounts of protein are quantified by suitable means, including width and intensity of stained bands, densitometry, bioactivity, and fluorescence. For transformed Bt cells or other cells known not to synthesize Bs binary toxin in their untransformed state, all production of Bs binary toxin is considered to represent enhancement by the methods of the invention. Where Bs cells are transformed with the nucleic acids of the invention, the net amount of toxin produced by the transformed cells can be compared to like untransformed cells. Net amount of toxin refers to the amount of Bs binary toxin in parasporal bodies or crystals. The control hosts are otherwise genetically identical with the transformed hosts and grown on comparative media. Enhancement is any statistically significant increase in Bs binary toxin production. In a preferred embodiment, parasporal bodies are isolated by centrifugation from lysed cultures and are examined by SDS-PAGE gels stained with Coomassie blue.

EXAMPLES Example 1 Expression Levels of Bs Binary Toxin Produced in Bti Using a Bti Promoter, STAB-SD Sequence, and Coding Sequence for the Toxin

A Bacillus sphaericus 2362 binary toxin gene was introduced into an acrystalliferous strain (4Q7) of Bacillus thuringiensis subsp. israelensis (Bti) using cyt1A promoters and a STAB-SD sequence placed into the plasmid pHT3101. The construct resulted in binary toxin production which appears to be 15-fold or more greater per unit of culture medium than that obtained with the parental (wild type) B. sphaericus strain grown on the same medium, as assessed by densiometric scanning of gels produced by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). TABLE 1 Yield Increases Obtained Using cyt1A Promoters and the STAB-SD Sequence to Drive Expression of the Bs 2362 Binary Toxin Gene Operon Increase in Decrease in Bti Strain Binary Toxin Toxins Bs 2362 (wild type)    1 — Bti IPS82 (Wild type) — 1 Bti4Q7/Bs Binary toxin >15× Bti IPS82 + Bs Binary toxin >20× .15-.35

Example 2 Toxicity of Non-Toxic Bti Engineered to Express Bs Binary Toxin

The toxicity of the acrystalliferous 4Q7 Bti strain, transformed to produce Bs 2362 binary toxin, was tested on fourth instar larvae (“L₄”) of Culex quinquefasciatus and compared to the wild type Bs 2362 strain grown on the same medium. (Bti strain 4Q7 does not normally produce Bs or Bti toxins.) LC₅₀ is the amount of toxin required to kill 50% of the larvae present in a sample during a test.

As shown on Table 2, below, the amount of wild-type Bs2362 needed to kill 50% (“LC₅₀”) of fourth instar larvae of Culex mosquitoes was 15.0 ng/ml. The 4Q7 Bti strain, transformed by nucleic acids of the invention to express Bs toxin, had an LC50 of 1.4, or approximately 10 times better toxicity than that of unaltered Bs.

Example 3 Toxicity of Bti Engineered to Express Bs Binary Toxin

Transformation of Bacillus thuringiensis subsp. israelensis with the plasmid described in Example 1 that produces the Bs2362 binary toxin increased toxicity by at least 10-fold against Culex species compared to either of the parental strains (Bs or Bti).

Bti IPS82 is the strain of Bti used as a commercial biopesticide. As can be seen from Table 2, the amount of this strain needed to kill 50% (“LC₅₀”) of fourth instar lavae (“L₄”) of Culex mosquitoes was 19.5 ng/ml. Wild-type Bs strain 2362 had an LC50 of 15 ng/ml. The Bti IPS82 strain, transformed by nucleic acids of the invention to express Bs toxin, had an LC50 of 1.5, or approximately 13 times better toxicity than that of unaltered Bs. TABLE 2 Toxicity of a Bti/Bs2362 Recombinant to L₄ Culex quinquefasciatuS Ratio Ratio Bti Bs Strain LC₅₀ (ng/ml) Bti/Bs Bti/Bs Bti IPS82 19.5 .1.0 — Bs 2362 15.0 —  1.0 Bti4Q7/Bs Binary toxin 1.4 — 10.0 Bti IPS82 + Bs Binary toxin 1.5 13.0 —

Example 4 Materials and Methods Used in Examples 1-3

A. Bacterial Strains, Gene, Plasmids and Transformation

Bacillus sphaericus strain 2362 was obtained from a powdered preparation that was kindly provided by Abbott Laboratories (North Chicago, Ill.). Escherichia coli-B. thuringiensis shuttle expression vector pHT3103 (Lereclus et al. FEMS Microbiol. Lett. 51:211-7 (1989)) was used to make and amplify the plasmid construct (pPHSP-1) in E. coli DH5α. The pPHSP-1 construct was expressed in an acrystalliferous strain, 4Q7, of B. thuringiensis subsp. israelensis obtained from the Bacillus Stock Center at Ohio State University (Columbus, Ohio), or in B. thuringiensis subsp. israelensis IPS82 (Abbott Laboratories). The modified pHT3101-based vector (pSTAB-SD) containing the 660-bp fragment with the cyt1A promoters and STAB-SD sequence (Agaisse and Lereclus, Mol. Microbiol., 20:633-643 (1996)) was previously described (Park et al., FEMS Microbiol Lett 181:319-327 (1999)). Plasmids were purified using the QIAprep Spin Miniprep Kit (Qiagen Inc.). Bacillus strains were transformed by electroporation as described by Park et al. App Environ. Microbiol 64:3932-3938 (1998).

B. PCR Amplification of the Gene Encoding the B. Sphaericus Entomocidal Proteins

A crude plasmid preparation was made from B. sphaericus 2362 using the alkaline lysis method (Sambrook et al., 1989). The gene encoding the 54.1 kDa protein and 41.9 kDA entomocidal protein of B. sphaericus (Baumann et al., J. Bacteriol. 170:2045-2050 (1988), GenBank M20390) was obtained by PCR using Vent (Exo+) DNA polymerase (Biolabs) and the primers BSP-1 (5′aactgcagCTTGTCAACATGTGAAGATTA AAGGTAACTTTCAG-3′) (SEQ ID NO:10) and BSP-2 (5′-aactgcagCCAAACAA CAACAGTTTACATTC GAGTGTAAAAGTTC-3′) (SEQ ID NO:11) (Genosys). The 3.4 kbp PCR product was digested with PstI and cloned in the same site in pHT3101 to generate pHBS. The 3.0 kbp HpaI-PstI fragment in pHBS was cloned into the filled XbaI and PstI sites in pSTAB-SD to generate pPHSP-1.

C. Growth of Bacterial Strains

The strains B. thuringiensis subsp. israelensis 4Q7/pPHSP-1 and B. thuringiensis subsp. israelensis IPS82/pPHSP-1 were grown on nutrient agar (BBL Microbiology Systems) or peptonized milk (1% peptonized milk [BBL Microbiology Systems], 1% dextrose, 0.2% yeast extract, 1.216 mM MgSO₄, 0.072 mM FeSO₄, 0.139 mM ZnSO₄, 0.118 mM MnSO₄) with erythromycin at a concentration of 25 μg/ml. For insect bioassays, B. thuringiensis subsp. israelensis IPS82/pPHSP-1 was grown in 25 ml of peptonized milk with erythromycin (25 μg/ml) in a shaker incubator set at 28° C., 250 rpm/min for 6 days, during which time >98% of the cells had sporulated and lysed. Spores and crystals were harvested by centrifugation at 4° C., 6,000×g for 15 min. The -pellet was washed twice in water and dried in a vacuum chamber.

D. Sodium Dodecyl Sulfate-Polyacrylamide Gel Electrophoresis (SDS-PAGE)

After 6 days growth in peptonized milk, 1 ml of the lysed culture was collected and centrifuged at 10,000×g for 5 min. The medium was discarded and 150 μl of TE buffer (10 mM Tris-Cl, pH 7.5, 1 mM EDTA) and 150 of 2× sample buffer (Laemmli, 1970) was added. Proteins were fractionated by SDS-PAGE (Laemmli, 1970).

E. Bioassays

For bioassays, groups of 20 early fourth instars were exposed to a range of concentrations of the lyophilized spore/crystal powders in 100 ml of deionized water held in 237 ml plastic cups. Seven to 9 different concentrations of the powders were replicated on 5 different days.

F. Microscopy

Sporulating cultures were monitored by light microscopy with a Zeiss Photomicroscope III, using a 100× oil immersion objective. For transmission electron microscopy, sporulated cells from peptonized milk cultures were collected just before lysis, fixed for 2 hr in 3% phosphate-buffered glutaraldehyde and 0.25% sucrose, post-fixed in 1% OSO₄, dehydrated in ethanol-propylene oxide, and embedded in Epon-Araldite (Ibarra and Federici, J. Bacteriol. 165:527-533 (1986)). Ultrathin sections of sporulated cells were examined and photographed in a Hitachi 600 electron microscope operating at an accelerating voltage of 75 kV.

Example 5 Cyt1A Protein Restores Susceptibility to BS Toxin to Mosquitoes Highly Resistant to that Toxin

A. Materials and Methods

Bacterial strains and toxins.

Toxin preparations used in this study were lyophilized powders of lysed cultures of B. sphaericus 2362 and a recombinant strain of B. thuringiensis subsp. israelensis that only produces Cyt1Aa (Wu and Federici, J. Bacteriol. 175:5276-5280 (1993)). These powders contained the spore and the crystal (that is, the parasporal body) along with cell debris and media solids resulting from lyophilization. The specific powders tested were (1) B. sphaericus strain 2362, obtained as a technical powder of the wild-type strain from Abbott Laboratories (North Chicago, Ill.); (2) Cyt1Aa, a recombinant strain of BTI noted above; and (3) BTI 4Q7, an acrystalliferous strain of this subspecies that does not produce any endotoxins. This strain was obtained from the Bacillus Stock Center (Ohio State University, Columbus, Ohio) and used as one of the controls. Lyophilized powders of purified Cyt1A crystals (Wu and Federici, supra) were also used.

Toxin powder production and storage.

Bacterial strains producing the various toxins were grown on solid or liquid media as described previously (Wirth et al., Proc Natl. Acad. Sci USA, 94:10536-10540 (1997), Park et al., Appl Environ Microbiol. 64:3932-3938 (1998)). The sporulated cells were washed in distilled water, sedimented, and the resultant pellet was lyophilized. For mosquito selections and bioassays, stock suspensions of the powders were prepared in distilled water and homogenized with the aid of approximately 25 glass beads. Stocks were prepared monthly and ten-fold serial dilutions were prepared weekly. All stocks and dilutions were frozen at −20° C. when not in use.

Mosquito strains.

Two strains of Cx. quinquefasciatus were used; BS-R, a strain resistant to B. sphaericus 2362, and Syn-P, an unselected, non-resistant strain. BS-R has been selected with B. sphaericus 2362 since 1992 and routinely survives 48 h of exposure to 1000 μg/ml, a concentration 149,000-fold higher than the concentration which kills 50% of Syn-P, the sensitive reference strain. Syn-P is a “synthetic” population of Cx. quinquefasciatus derived from larval populations collected in 1995 from 3 different geographic areas in southern California. This colony has been maintained in the laboratory without exposure to B. sphaericus.

Selection and bioassay procedures.

As noted above, the BS-R strain has been maintained under selection pressure with B. sphaericus 2362 since 1992. Selection consisted of exposing groups of ca. 1,000 early fourth-instars to concentrations of B. sphaericus ranging between 100-120 μg/ml in enameled metal pans in about 1 L of deionized water for 48-96 h. Average mortality of the larvae under selection was 10% or less per selection, and the survivors were used to continue the colony.

For bioassays, groups of 20 early fourth instars were exposed to a range of concentrations of the lyophilized spore/crystal powders in 100 ml of deionized water held in 237 ml plastic cups. Seven to 9 different concentrations of the powders, which yielded mortality between 2 and 98% after 48 h, were replicated on 5 different days. For the bioassays in which different combinations of Cyt1A and B. sphaericus 2362 were tested, different ratios of these toxins were based on the weights of the lyophilized powders of the bacterial strain.

Because the quantity of purified Cyt1A crystals was limited, bioassays with this powder utilized 10 early fourth instars held in 10 ml of deionized water in 30 ml plastic cups and replicated on 2-3 different days. Bioassays combining B. sphaericus 2362 technical powder and Cyt1A purified crystals at a 10:1 ratio (10 parts B. sphaericus 2362: 1 part Cyt1A crystal) were based on the weights of the lyophilized powders of B. sphaericus 2362 and Cyt1A.

All data were subjected to probit analysis using a program for the PC. Dose-response values with overlapping fiducial limits were not considered to be significantly different. Resistance ratios were calculated by dividing the respective lethal concentration value for the BS-R strain by that of the Syn-P strain. Resistance ratios whose fiducial limits contained the number 1 were not considered to be significant.

Evaluation of synergism.

Synergistic interactions between B. sphaericus 2362 and Cyt1A were evaluated using the method of Tabashnik, Appl Environ Entomol 58:3343-3346 (1992). Theoretical lethal concentration values for the different mixtures of Cyt1A and B. sphaericus 2362 were calculated from the weighted harmonic means of the individual values for these toxins. Because the B. sphaericus 2362 powder was not toxic to the BS-R strain at any of the concentrations tested, the calculation of the theoretical toxicity of a combination of Cyt1A and B. sphaericus 2362 was based on the toxicity and proportion of Cyt1A alone for this strain. The synergism factor (SF), defined as the ratio of the theoretical lethal concentration value to the observed lethal concentration value, was determined for combinations of B. sphaericus 2362 and the Cyt1A strain as well as for combinations of B. sphaericus 2362 and purified Cyt1A crystals. When the ratio was greater than 1, the toxin interaction was considered synergistic because toxicity exceeded the value predicted from individual additive toxicity. When the ratio was less than 1, the interaction was considered antagonistic, whereas a ratio of 1 indicated that the values were additive.

B. Results

In the bioassays to determine toxin baseline values under standard conditions against the resistant and sensitive mosquito strains, no mortality resulted from exposure of BS-R, the resistant strain of Cx. quinquefasciatus, to 1000 μg/ml of B. sphaericus 2362. This concentration was 149,000 fold higher than the LC₅₀ (0.0067 μg/ml) obtained against Syn-P, the sensitive strain. When the bioassays were carried out in 10 ml of water with 10 larvae per cup rather than 20 larvae in 100 ml, no mortality was obtained against BS-R, but the toxicity of BS 2362 was lower (LC₅₀, 0.032 μg/ml) against Syn-P. Increasing larval density has been previously shown to require lower amounts of Bti toxin to induce the same level of mortality observed at lower densities (Aly et al. 1988). The estimated difference in the sensitivity of BS-R and Syn-P using the smaller bioassay system was 31,000 fold.

The Cyt1A bacterial strain was slightly less toxic to the BS-R strain (LC₅₀, 32.5 μg/ml) than to Syn-P (LC₅₀, 11.7 μg/ml) in the standard bioassay system. However, in the tests using Cyt1A crystals in the smaller bioassay system, no difference in sensitivity (LC₅₀s, ca. 20 μg/ml) was observed between BS-R and Syn-P.

Adding Cyt1A to the B. sphaericus 2362 preparations restored most of its toxicity against the BS-R resistant Cx. quinquefasciatus strain. A B. sphaericus 2362 ratio to Cyt1A of 10:1 was highly toxic to both the resistant and sensitive mosquito strains. Toxicity levels for this combination were higher against Syn-P than BS-R, with LC₉₅ values of 0.442 and 36.6 μg/ml, respectively, and a resistance ratio (LC₉₅) of 82.9 for BS-R. The 5:1 ratio was more toxic toward Syn-P and BS-R, and the resistance ratio at the LC₉₅ level was reduced to 34.4-fold. At a ratio of 3:1 B. sphaericus 2362:Cyt1A, the mixture was again significantly more toxic to BS-R (LC₅₀, 1.99 μg/ml), and the resistance ratio decreased to 15.4 fold at the LC₉₅ level. Toxicity at a 1:1 ratio against BS-R was not significantly different from that of the 3:1 ratio. Overall, as the proportion of B. sphaericus 2362 to Cyt1A was increased, the toxicity increased toward both the resistant and sensitive mosquito strains. However, the resistance ratios at the LC₉₅ values for BS-R declined to insignificant levels for ratios of 1:3, 1:5, and 1:10, in which Cyt1 A was the principal component.

Calculation of the SF for these combinations revealed significant synergism between Cyt1A and B. sphaericus 2362 against the BS-R strain, but not against Syn-P. SF values ranged from 10-137 at the LC₉₅ level for BS-R. The highest levels of synergism were observed in the combinations in which Cyt1A was present in the lowest proportion (10:1, 5:1, 3:1). These combinations were antagonistic toward Syn-P at the LC₉₅ level at ratios 1:10, 1:5, and 1:3, and additive or mildly synergistic at ratios of 1:1, 3:1, 5:1, and 10:1, i.e., where B. sphaericus became the predominant component.

Bioassays using B. sphaericus 2362 combined with the purified Cyt1A crystals at a ratio of 10:1 demonstrated that this combination was highly toxic to both BS-R (LC₉₅, 4.96 μg/ml) and Syn-P (LC₉₅, 2.37 μg/ml). Although the BS-R strain was slightly less sensitive to the mixture, the toxicity values were not significantly different. Importantly, no resistance was detected against the BS-R strain with this combination, which had a high SF value of 278.

C. Discussion

Combining Cyt1A with BS 2362 restored the toxicity of the latter against a highly resistant strain of Cx. quinquefasciatus. Moreover, we were able to completely restore toxicity with sublethal concentrations of Cyt1A crystals, and therefore suppress resistance to B. sphaericus in the BS-R mosquito strain. In contrast to the high level of activity observed against the resistant mosquito population, little or no enhanced activity resulted with these same mixtures against the non-resistant reference strain, Syn-P.

The ability of Cyt1A at low concentrations to restore high toxicity to B. sphaericus 2362 against resistant mosquitoes has practical implications for control of Culex populations and provides insight into its mode of action. Bacterial larvicides based on B. sphaericus are used in several countries and resistance in field populations of Cx. quinquefasciatus has already been reported in France, Brazil, and India. The results of our studies indicate that adding Cyt1A at a ratio as low as 1:10 to B. sphaericus larvicides restores most of the toxicity against even highly resistant populations of Cx. quinquefasciatus. Therefore, Cyt1A provides a practical tool for managing B. sphaericus resistance. Furthermore, adding a small quantity of Cyt1A to B. sphaericus preparations can delay resistance in mosquito populations in which it has not already developed.

Others have shown that a different Cyt protein, Cyt1Ab from B. thuringiensis subsp. medellin, can suppress resistance to B. sphaericus 2297, a mosquitocidal strain of this bacterium that produces a large toxin crystal, in Cx. pipiens (Thiery et al. 1998). However, Cyt1Ab's suppression of resistance to B. sphaericus 2297 was much less effective than Cyt1A's suppression of resistance to B. sphaericus. The reduced capacity of Cyt1Ab to suppress resistance to B. sphaericus 2297 may be due to the 5-fold lower toxicity of this Cyt toxin to Cx. pipiens in comparison to Cyt1A (Thiery et al. Appl Environ Microbiol 63:468-473 (1997)).

Just how Cyt1A restores the toxicity of B. sphaericus 2362 is unknown. However, previous studies of the mechanism of resistance in our BS-R strain of Cx. quinquefasciatus and Cyt1A's binding properties suggest that Cyt1A assists binding and insertion of the toxin into the microvillar membrane. Our resistant strain of Cx. quinquefasciatus has no functional receptor for the B. sphaericus 2362 toxin and therefore it cannot bind effectively to the midgut microvilli. Studies of Cyt1A have shown that it perturbs membranes by binding to the lipid portion, and that it also binds to Cry toxins. Moreover, in the presence of the BTI Cry toxins, Cyt1A binds to the microvilli of cells in the gastric caeca and posterior midgut of mosquito larvae. These observations suggest several mechanisms for restoring B. sphaericus toxicity. The Cyt1A and B. sphaericus toxins may bind together after dissolution, and then insert into the membrane as a complex due to Cyt1A's lipophilic properties. Another possibility is that Cyt1A may first bind to the membrane after which the B. sphaericus toxin binds to Cyt1A and inserts into the membrane. Finally, Cyt1A may permeate the membrane causing lesions that allow the B. sphaericus toxin to gain access to the original target.

The synergism we obtained with the combinations of Cyt1A and B. sphaericus 2362 also provides additional evidence that Cyt1A enhances toxicity by assisting other protein toxins in binding to the mosquito microvillar membrane, especially those that do not bind efficiently. In previous studies we demonstrated that Cyt1A can synergize Cry4 and Cry11 toxins from mosquitocidal strains of B. thuringiensis against resistant mosquitoes. However, synergism in non-resistant mosquitoes was observed only with the Cry4 and Cry11A toxins of BTI, not with the Cry11B toxin from B. thuringiensis subsp. jegathesan, which is much more toxic than Cry11A. A similar pattern of synergism was observed in the current study wherein Cyt1A synergized the toxicity of B. sphaericus 2362 against the resistant BS-R strain, but not against the sensitive Syn-P strain. The implication of these results, in conjunction with those obtained in the previous studies cited above, is that toxins which are highly toxic or have a high binding affinity, such as Cry11B or the B. sphaericus 2362 binary toxin, gain little or no value from assisted binding by Cyt1A. But when the toxin receptors are modified or lost through resistance, Cyt1A's ability to bind to and perturb the microvillar membrane restores the capacity of these toxins to insert into the membrane and exert toxicity. As both the Cyt1A and B. sphaericus toxins dissolve in the mosquito midgut lumen, they may associate immediately after dissolution in the lumen as well as at the microvillar membrane surface. An implication of these results is that Cyt1A, and possibly other Cyt proteins, may extend the insecticidal spectrum of non-Cyt protein toxins to other insect species. TABLE 3 Toxicity of B. sphaericus (strain 2362) technical powder, Cyt1A crystal/spore powder from B. t. subsp. israelensis, and various combinations of B. sphaericus and Cyt1A against susceptible (Syn-P) and B. sphaericus resistant (BS-R) C. quinquefasciatus LC₅₀ (μg/ml) LC₉₅ (μg/ml) Slope Resistance ratio at SF Toxin(s) Strain No. (fiducial limits) (fiducial limits) (±SE) x² LC₅₀ (FL) LC₉₅ (FL) LC₅₀ LC₉₅ B. sphaericus Syn-P 1,100 0.00671 0.466 0.89 13.1 1.0 1.0 (strain 2362) (0.0055-0.0082) (0.300-0.790) (0.045) BS-R 600 No mortality at ˜149,000 1,000 μg/ml Cyt1A Syn-P 600 11.7 59.8 2.3 7.3 1.0 1.0 (10.2-13.4) (47.7-79.7) (0.16) BS-R 700 32.5 222 2.0 4.1 2.7 3.7 (28.3-37.6) (172-304) (0.12) (2.3-3.3) (2.6-5.3) B. sphaericus + Syn-P 900 0.0288 0.0422 1.4 22.8 1.0 1.0 0.26 1.2 Cyt1A (10:1)^(a) (0.0163-0.0508) (0.162-1.23)  (0.21) BS-R 800 2.47 36.6 1.4 25.4 85.8 82.9 132 61 (1.46-4.20) (14.0-97.4) (0.17) (56.8-129)   (39-174) B. sphaericus + Syn-P 700 0.0274 0.278 1.6 2.4 1.0 1.0 0.29 2.0 Cyt1A (5:1) (0.0232-0.0322) (0.209-0.397) (0.10) BS-R 1,000 1.23 9.58 1.8 12.5 45.0 34.4 155.9 136.8 (1.05-1.43) (7.49-12.9) (0.11) (38.1-53.2) (25.2-46.9) B. sphaericus + Syn-P 800 0.0147 0.652 1.0 27.1 1.0 1.0 0.6 1.0 Cyt1A (3:1) (0.0086-0.0354) (0.177-2.48)  (0.12) BS-R 600 1.99 7.17 2.9 6.0 297 15.4 65 124 (1.80-2.22) (5.87-9.31) (0.22) (255-347) (10.9-1.7) B. sphaericus + Syn-P 1,000 0.0381 0.464 1.5 10.1 1.0 1.0 0.35 2.0 Cyt1A (1:1) (0.0323-0.0449) (0.348-0.655) (0.08) BS-R 1,000 0.735 6.49 1.7 5.8 19.3 14.0 88 69 (0.632-0.853) (5.06-8.73) (0.09) (16.5-22.5) (10.5-18.7) B. sphaericus + Syn-P 900 0.234 7.54 1.1 11.5 1.0 1.0 0.11 0.24 Cyt1A (1:3) (0.191-0.287) (5.00-12.5) (0.06) BS-R 900 1.71 18.4 1.6 6.5 7.3 2.4 25 16 (1.45-2.00) (14.1-25.5) (0.09) (6.3-8.5) (1.8-3.2) B. sphaericus + Syn-P 1,000 0.189 6.74 1.1 13.5 1.0 1.0 0.21 0.39 CytA (1:5) (0.149-0.236) (4.66-10.6) (0.06) BS-R 900 1.56 11.8 1.9 8.9 8.2 1.8 25.3 23.0 (1.34-1.81) (9.23-15.9) (0.11) (6.9-9.6) (1.3-2.3) B. sphaericus + Syn-P 900 1.06 25.9 1.2 4.8 1.0 1.0 0.10 0.17 Cyt1A (1:10) (0.859-1.29)  (18.1-40.1) (0.07) BS-R 900 4.72 24.6 2.3 13.0 4.4 1.0 7.7 10.0 (4.12-5.38) (19.8-32.0) (0.15) (3.7-5.2) (0.69-1.3)  SF, synergism factor. ^(a)Ratios in brackets represent the relative proportion of B. sphaericus technical powder to Cyt1A spore/crystal powder (BS:CytA). All ratios were based on the weight of each respective powder.

All publications and patent applications cited in this specification are herein incorporated by reference as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference.

Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, it will be readily apparent to one of ordinary skill in the art in light of the teachings of this invention that certain changes and modifications may be made thereto without departing from the spirit or scope of the appended claims. 

1. A nucleic acid sequence comprising, in the following order, a B. thuringiensis cyt promoter selected from the group consisting of a BtI promoter, a BtII promoter, and a combination of a BtI and a BtII promoter, a bacterial STAB-SD sequence, a ribosome binding site, and a sequence encoding one or both proteins of a B. sphaericus binary toxin.
 2. The method of claim 1, wherein the B. thuringiensis promoter is selected from the group consisting of cyt1Aa1, cyt1Aa2, cyt1Aa3, and cyt1Aa4.
 3. The nucleic acid of claim 2, wherein the B. thuringiensis promoter is a cyt1Aa1 promoter.
 4. The nucleic acid of claim 1, having a BtI promoter and a BtII promoter, wherein the BtI promoter and the BtII promoter are overlapping.
 5. An expression vector comprising a nucleic acid of claim
 1. 6. An expression vector comprising a nucleic acid of claim
 2. 7. An expression vector comprising a nucleic acid of claim
 3. 8. An expression vector comprising a nucleic acid of claim
 4. 9. A host cell comprising an expression vector of claim
 5. 10. A host cell comprising an expression vector of claim
 6. 11. A host cell comprising an expression vector of claim
 7. 12. A host cell comprising an expression vector of claim
 8. 13. A host cell of claim 9 further comprising a cry11A 20 kD protein.
 14. A host cell of claim 10 further comprising a cry11A 20 kD protein.
 15. A host cell of claim 11 further comprising a cry11A 20 kD protein.
 16. A host cell of claim 12 further comprising a cry11A 20 kD protein.
 17. A host cell of claim 9, wherein the cell is a B. thuringiensis cell.
 18. A host cell of claim 10, wherein the cell is a B. thuringiensis cell.
 19. A host cell of claim 11, wherein the cell is a B. thuringiensis cell.
 20. A host cell of claim 12, wherein the cell is a B. thuringiensis cell.
 21. A method of creating a recombinant bacterium, said method comprising the steps of: (a) transforming the recombinant bacterium with a gene comprising, in the following order: a B. thuringiensis promoter selected from the group consisting of a BtI promoter, a BtII promoter, and a combination of a BtI and a BtII promoter, a bacterial STAB-SD sequence, a ribosome binding site, and a sequence encoding one or both proteins of a B. sphaericus binary toxin; and (b) expressing said gene in the host cell; whereby expression of said gene enhances production of B. sphaericus binary toxin as compared to production of B. sphaericus binary toxin in a wild-type B. sphaericus cell that is not transformed with said gene.
 22. The method of claim 21, wherein the recombinant bacterium is selected from the group consisting of B. thuringiensis, B. sphaericus, and a member of a Bacillus species other than Bti or Bs.
 23. A method of increasing toxicity of a B. thuringiensis bacterium to a mosquito, said method comprising the steps of: (a) transforming said bacterium with a nucleic acid sequence comprising, in the following order, a B. thuringiensis promoter selected from the group consisting of a BtI promoter, a BtII promoter, and a combination of a BtI and a BtII promoter, a bacterial STAB-SD sequence, a ribosome binding site, and a sequence encoding one or both proteins a B. sphaericus binary toxin; and (b) expressing said gene in the bacterium; whereby expression of said gene enhances production of B. sphaericus binary toxin as compared to production of B. sphaericus binary toxin in a wild-type B. sphaericus cell that is not transformed with said gene.
 24. A method for suppressing resistance to Bacillus sphaericus binary toxin, said method comprising expressing a Bacillus thurengiensis israelisis (Bti) Cyt1Aa1 protein in a B. thuringiensis cell expressing said binary toxin 